Development of depth-resolved light reflection or Optical Coherence Tomography (OCT) provides a high resolution imaging technique for analyzing tissue. OCT is an imaging technique that captures micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). OCT uses a narrow line width tunable laser source or a superluminescent diode source to emit light over a broad bandwidth. The light is sent to an interferometer that includes a sample arm and a reference arm. A majority of the light is sent to the sample arm to be directed onto a sample, while the remainder of the light is sent to a reference arm. The light reflects from both the sample and the reference arms and is recombined to make in situ tomographic images. Those images typically have an axial resolution of less than 10 μm and tissue penetration of 2-3 mm. OCT provides tissue morphology imagery at much higher resolution than other imaging modalities such as MRI or ultrasound. Further, with such high resolution, OCT can provide detailed images of a pathologic specimen without cutting or disturbing the tissue.
Conventional, intensity based OCT cannot directly differentiate between tissues. However, the light's polarization state can be changed by various light-tissue interactions, allowing it to be used to generate tissue specific contrast. Those effects are used by polarization sensitive (PS) OCT. With PS-OCT, a sample is typically illuminated either with circularly polarized light or with different polarization states successively, and the backscattered light is detected in two orthogonal polarization channels.
With polarization diverse detection, optical field intensity of the reference arm of the interferometer needs to be approximately equalized between two orthogonal detection channels. Using single mode fiber in the reference arm causes its light to propagate in an uncontrolled polarization state. To address that concern, a polarization controller is typically used to adjust the polarization state to have equalized intensities in the detection channels. The polarization control component is typically somewhat bulky, it must be adjusted for every system produced, and its control function must be automated (with sensor feedback) if thermal changes in the fiber polarization state are expected. For those reasons, a polarization controller is disadvantageous.
Polarization maintaining optical fiber is an option that maintains the polarization state of light as it propagates in the fiber and therefore an external polarization control component is not needed. However, use of polarization maintaining fiber has other disadvantages including higher cost, tight angular tolerances on splices/connectors, non-zero crosstalk between polarization modes, and the inherent polarization specific differential group delay incurred in the light propagating in the two polarization modes within the fiber.